1. Field of the Invention
The invention is in the field of electrical power production from high temperature geothermal brines which have a high dissolved silica content.
2. Description of the Prior Art
Some very high temperature geothermal brine resources are known which contain large amounts of geothermal energy, but which have heretofore not been usable for the commercial production of electrical power because of a high dissolved silica content. These resources include the Salton Sea and Brawley geothermal fields in the Imperial Valley in California, where underground geothermal brine source temperatures range from about 500.degree. F. to about 620.degree. F. and the dissolved silica content is at substantially the saturated level for these brine source temperatures, ranging from about 500 to about 600 parts per million by weight. This high silica content appears to result from the nature of the porous rock formations in the high temperature brine sources in geothermal fields of this type.
Extensive flow testing of high temperature, high silica Salton Sea field brines has been carried out in a simulated geothermal power plane at Niland, Imperial County, California, where the high temperature geothermal brine is flowed up through a production well under the power of its own flashing steam, then flowed through a heat extraction facility simulating a geothermal electrical power generating plant, and then pumped back into an underground formation through a reinjection well spaced a considerable distance from the production well. This testing operation at Niland, although making use of the best state-of-the-art technology, revealed that the high, substantially saturated dissolved silica content in this type of high temperature geothermal brine resource causes such a severe plugging problem in reinjection equipment, and particularly in the reinjection well, that it would not be practical to attempt to produce electrical power on a commercial basis from this type of geothermal resource using prior art technology.
The basis for this problem is that the saturation amount of silica lowers as the temperature of the geothermal brine drops when the brine flows up to the production well, passes through the heat extraction equipment of the plant, and is then flowed through piping to a remote reinjection well and down into the reinjection well. The silica saturation curve has a relatively gentle descent from the underground source temperature of approximately 500.degree. F. to 620.degree. F. down through relatively high temperature heat extraction equipment of the plant such as a first stage flash vessel in which the temperature of the brine may be reduced down to perhaps 320.degree. F. to 375.degree. F. after flashing. However, the silica saturation curve then drops off much more steeply below these temperatures, and particularly in reinjection equipment including the reinjection piping and reinjection well wherein the brine temperature will drop down to approximately the boiling point of the brine.
Because the silica saturation curve descends only gradually in the temperature range of the production well bore, and the silica precipitation reaction is a slow one, little or no silica scaling is likely to occur in the production well bore. This also holds true for production brine input conduits of the plant where the brine temperature is still high and residency time is brief. However, the increasingly steep silica saturation curve for the relatively large temperature drop in successive flash vessels or other heat extraction apparatus such as heat exchangers in a binary system, and the further temperature drops associated with reinjection piping and initial flow into a reinjection well, coupled with a considerable residency time of the brine in such heat extraction and reinjection equipment, will cause silica to precipitate out of the brine in progressively increasing quantities as the temperature of the brine is reduced in its flow path through the heat extraction and reinjection equipment. The amount of silica precipitation is greatest in the reinjection equipment, both because of the fact that the silica saturation level drops at the greatest rate as the temperature of the brine approaches reinjection temperatures, and also because the slow silica precipitation reaction has, by the time the brine reaches the reinjection equipment, already had a considerable amount of time to operate in the supersaturated condition upstream of the reinjection equipment.
Such silica precipitation occurs principally as scaling in the form of a hard, rock-like glaze on the inner walls of flash vessels or heat exchangers in the heat extraction apparatus, in various conduits including the usually quite long reinjection conduit from the main plant site to a reinjection well site, and most importantly in the reinjection well casing where the precipitated silica causes rapid plugging that will lead to loss of the well. While surface equipment of the plant such as steam separators and piping can either be cleaned out or replaced when the silica buildup becomes too great, silica precipitation that causes plugging within a reinjection well casing is, under the present state of the art, a much more serious problem in that there is no practical conventional way to clean out such a silica-plugged reinjection well, so that when such plugging reduces the reinjection flow volume below acceptable limits, it would then be necessary to drill and case a new reinjection well, at great expense.
Some attention was given to the matter of controlling mineral precipitation in connection with a geothermal electrical power plant in U.S. Pat. No. 3,757,516 issued to Barkman C. McCabe. That patent taught the principle of deep well pumping in a geothermal hot water production well and pressurization throughout the entire plant system on through reinjection primarily to avoid the loss of heat of vaporization from that portion of the fluid which would otherwise flash to steam in the production well, but also to prevent mineral precipitation at any point in the entire flow path. However, the said McCabe Pat. No. 3,757,516 was concerned only with geothermal plants that produced electrical power from geothermal resources having temperatures under about 400.degree. F., where the principal scaling problem involved the precipitation of carbonates in the production well bore and heat exchangers.
The pressurization procedure taught in said McCabe U.S. Pat. No. 3,757,516 while effective to prevent some types of scaling such as from carbonates, would, however, not be effective to prevent dissolved silica from precipitating out in the reinjection piping and reinjection well of a geothermal power plant, as such silica precipitation depends only upon temperature reduction to put the dissolved silica in a supersaturated condition, and time for the slow silica precipitation reaction to occur. Nevertheless, the geothermal hot water or brine having the relatively low source temperatures to which that prior patent applied (below about 400.degree. F.) did not carry the large quantities of silica (even if saturated with silica) that are found at substantially the saturation level in very hot brines running from about 500.degree. F. to about 620.degree. F. in geothermal energy resources like the Salton Sea and Brawley geothermal fields. It is these large quantities of silica in very hot brine which heretofore have presented insurmountable problems in attempts to utilize this huge thermal potential energy resource for the commercial generation of electrical power.
U.S. Pat. No. 4,043,129 issued to Barkman C. McCabe and Edward Zajac applied the deep well pumping concept of the earlier McCabe U.S. Pat. No. 3,757,516 to high temperature geothermal brines above about 500.degree. F. The McCabe-Zajac U.S. Pat. No. 4,043,129 taught that the advantages of deep well pumping, including avoidance of the loss of heat of vaporization and reduction of mineral precipitation, could be realized in connection with very high temperature geothermal brines by mixing a high temperature geothermal brine which might have a relatively high mineral content derived from a relatively deep well with a lower temperature brine that might have a relatively low mineral content derived from a shallower or peripheral well, to produce a brine mixture of sufficiently lower temperature to the within the practical temperature and pressure ranges for pumping and which may also have a lowered mineral content. However, this still would not solve the serious silica scaling problem in the reinjection piping and reinjection well of a geothermal power plant where the high temperature brines had a dissolved silica content proximate saturation levels at source temperatures, the situation in the Salton Sea and Brawley geothermal fields.
Some more recent prior art work has involved the use of a reactor clarifier, a type of apparatus known in the sewage treatment art, in an attempt to reduce the silica content of high temperature, high silica geothermal brines so as to protect reinjection well equipment against silica plugging. A publication regarding such use of a reactor clarifier was made at the Second Invitational Well-Testing Symposium of Oct. 25-27, 1978 at the University of California, Berkeley, by Robert H. Van Note, John L. Featherstone and Bernard Pawlowski, entitled "A Cost-Effective Treatment System for the Stabilization of Spent Geothermal Brines". In the normal mode of operation of a reactor clarifier for the treatment of sewage it was left open for exposure of the sewage to atmospheric oxygen; there was no suggestion in said publication that anything other than such normal mode of operation be employed when a reactor clarifier was used for the treatment of geothermal brine. There was no concern in the prior art respecting exposure to atmospheric oxygen of geothermal brines from such sources as the Salton Sea and Brawley geothermal fields, because it was well recognized in the prior art that with the very high chloride salt contents of such geothermal brines (from about 200,000 to about 360,000 parts per million by weight of chloride salts after the brine had been flashed to atmospheric pressure) the brines had little or no capacity for the absorption of atmospheric oxygen.
However, geothermal brines from such sources as the Salton Sea and Brawley geothermal fields have a peculiar combination of chemical characteristics that applicants have found would cause very serious problems if the brines should be exposed to atmospheric oxygen, although there was no recognition of such in the prior art. These brines in their production source condition are in a very reduced oxygen state, and they contain a large amount of dissolved iron in its chemically reduced ferrous (Fe.sup.2+) state. Despite the normal incapacity of brines having such high chloride salt content to absorb oxygen, the very reduced state thereof enables a chemical hydrolysis reaction to occur which involves atmospheric oxygen and ferrous iron. This results in a considerable amount of iron precipitating out which increases reinjection well plugging problems, and also results in a reduction of the pH of the brine which makes it more acidic and corrosive to reinjection equipment.